1. Field of the Invention
The present invention relates to a circuit that detects a voltage between arbitrary terminals that operate at a low voltage.
2. Description of the Related Art
As a voltage detecting circuit, there has been known a circuit shown in a circuit block diagram of FIG. 6 (for example, refer to JP-A 1-245165, pages 1 to 3 and FIG. 2). That is, terminals 11 and 10 are connected with arbitrary terminals to be detected. In FIG. 6, a battery 1 is connected to the terminals 11 and 10 so that a voltage across the battery 1 is detected. Voltage division resistors 13 and 14 are connected between the terminals 11 and 10. An input of a comparator 16 is connected with a node of the voltage division resistors 13 and 14, and with a reference voltage 15. An output of the comparator 16 is connected with an inverter circuit 17. An output of an output circuit 20 is connected to an output terminal 12.
The output circuit 20 is structured by an inverter circuit which is made up of a p-channel MOS transistor (hereinafter referred to as “p-ch MOS Tr”) 18 and an n-channel MOS transistor (hereinafter referred to as “n-ch MOS Tr”) 19.
The voltage detection is conducted by comparing a voltage Va at the node of the voltage division resistors 13 and 14 with a voltage Vb of the reference voltage 15 through the comparator 16. That is, a voltage at which the comparator 16 is inverted is Va=Vb. In this example, Va changes due to the values of the voltage division resistors 13 and 14, and the voltage across the battery 1. Assuming that the resistance of the resistor 13 is R1, the resistance of the resistor 14 is R2, and the voltage across the battery 1 is V1, the detected voltage across the battery 1 is represented by the following expression (1) from Va=R2/(R1+R2)×V1=Vb.Detected Voltage=(R1+R2)/R2×Vb  (1)
That is, when the voltage across the battery 1 is higher than the voltage obtained from the expression (1), the output of the comparator 16 becomes high level, and when the voltage across the battery 1 is lower than the voltage obtained from the expression (1), the output of the comparator 16 becomes low level. The output of the comparator 16 passes through the inverter circuit 17 and the output circuit 20 that constitutes the inverter and is then outputted to the output terminal 12. In other words, it is possible to detect whether the voltage across the battery 1 is higher or lower than the detected voltage in accordance with the output of the comparator 16 being high level or low level.
In general, as shown in FIG. 6, the output of the output circuit that constitutes the inverter becomes unstable at a voltage equal to or lower than its operation voltage. This is because both of the p-ch MOS Tr 18 and n-ch MOS Tr 19 which constitute the output circuit 20 are enhancement-type transistors, and therefore the transistors cannot be turned on unless a voltage equal to or higher than a threshold voltage of the transistors is applied between the gate and source of the transistors.
That is, because both of the p-ch MOS Tr 18 and the n-ch MOS Tr 19 are in an off-state at the voltage equal to or lower than the threshold voltage of the transistors, the voltage of the output terminal 12 becomes unstable.
FIG. 7 shows a voltage V12 of the output terminal 12 in an ordinate axis when the voltage V1 across the battery voltage 1 of the conventional voltage detecting circuit shown in FIG. 6 is changed as an abscissa axis. The voltage at the output terminal 12 is divided into the L level and the H level with the boundary of the detected voltage. In a region A of FIG. 7, the output of the output terminal 12 must be low voltage level (hereinafter referred to as “L level”) because the voltage at the output terminal 12 is naturally the battery voltage lower than the detected voltage. However, because the voltage is lower than the threshold voltage, the n-ch MOS Tr 19 shown in FIG. 6 cannot turn on, and the output terminal 12 cannot output the L level.
In other words, when the voltage detecting circuit is used for a reset signal of an arbitrary system, in the case where the detected voltage is low, the system side judges that the voltage detecting circuit outputs H level in the region A of FIG. 7, with the result that there is a fear that the system malfunctions.
The conventional voltage detecting circuit suffers from such a problem that the output voltage of the voltage detecting circuit becomes inconstant at the operation voltage or lower, thereby making it impossible to provide an accurate voltage detection output.